• The Islamic State claimed responsibility on Tuesday for the Easter Sunday bombings at churches and hotels in Sri Lanka, as the government there raised the number of people killed to 321. The group’s Amaq news agency called the bombers “Islamic State fighters.”

• The government said the bombings might have been in retaliation for the killing of 50 people last month at mosques in New Zealand, and that two Islamist extremist groups might have been involved, not one.

• In his first national address since the attacks, President Maithripala Sirisena announced major changes to the country’s security apparatus. “I must be truthful and admit that there were lapses on the part of defense officials,” Mr. Sirisena said.

• The F.B.I. has joined the investigation, officials said, as recriminations continued over the Sri Lanka government’s failure to act on warnings that terrorists were planning to attack churches. The country’s highest Roman Catholic prelate joined those chastising the government.

• The first mass funerals were held at a damaged church in western Sri Lanka, where as many as 100 people were killed. Many of the coffins bore the remains of children. [Read about some of the victims.]

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Mourners at the mass funeral at St. Sebastian’s Church in Negombo on Tuesday.CreditAdam Dean for The New York Times

Islamic State says it was behind the bombings

More than two days after the Sri Lanka bombings, the Islamic State claimed responsibility for the coordinated attacks.

The group’s news agency, Amaq, released a bulletin on Tuesday claiming they had been carried out by “Islamic State fighters.” The statement, which was disseminated on the group’s chat rooms on the app Telegram, said that the bombings had been intended to target Christians, as well as citizens of countries belonging to the coalition fighting the Islamic State, also known as ISIS.

In a later statement, the group identified the seven suicide bombers by their noms de guerre, and specified which of them had gone to which target on what it called “the infidel holiday.”

Still later on Tuesday, Amaq published a video showing eight men — apparently the Sri Lanka attackers — pledging allegiance to Abu Bakr al-Baghdadi, the leader of ISIS, whom they called “emir of the believers.” They held their hands up, index fingers pointed skyward, in a gesture that the Islamic State has adopted to refer to belief in one God.

The men are masked, except one who appears to be Mohammed Zaharan, who has been identified by Sri Lankan officials as the leader of the group that carried out the bombings.

A day earlier, an ISIS-linked chat room on the app Telegram had posted photos of masked men it identified as “commandos” involved in the bombings. It used three noms de guerre that match three of the names used in the militants’ statement.

If it is authentic, the video indicates that the cell in Sri Lanka took steps to ensure that its violence would seen as being committed on behalf of ISIS, which has repeatedly called for assaults on churches, particularly since the New Zealand mosque attacks.

It does not necessarily mean there was direct communication or guidance from the terrorist group, but it indicates that the assailants were steeped in its ideology, and familiar with the terminology and rituals of carrying out an attack in ISIS’ name.

If true, the claim of Islamic State responsibility demonstrates that the group still poses a threat, despite the loss of the territory it once held in Syria and Iraq.

The attackers got help while overseas, and some were in Syria

“There seems to have been foreign involvement” in the bombings, Prime Minister Ranil Wickremesinghe said at a news conference, and investigators believe that some of the attackers “have traveled abroad and have come back.”

He added that some of the attackers, including one or two of the suicide bombers, had been in Syria. But he stopped short of saying that they had fought for the Islamic State, adding that their time in Syria was not necessarily the source of the bombing plot.

“You can meet people in any part,” he said. “You can meet them in London. You needn’t go to the region.”

“We can’t tell you immediately, definitively to whom they had links,” he said, but from the start, “there was suspicion that there were links with ISIS.”

Now, he added, “some of the evidence points to that.”

The prime minister said several countries were aiding in the investigation, and the United States ambassador to Sri Lanka, Alaina B. Teplitz, said that the F.B.I. had joined it.

A funeral service at St. Sebastian’s Church.CreditAdam Dean for The New York Times

Tiny coffins laid to rest in mass burial

The coffins came one by one, some heavy and others much lighter.

As bulldozers cleared more space in a vacant lot near St. Sebastian’s Church in Negombo, barefoot men dripping with sweat scooped dirt with shovels in punishing heat.

One family stood in the shade. They were there for the burial of an 11-year-old boy.

“I don’t even know what to say,” said Lasanthi Anusha, a woman who came for the burial of her son’s classmate. “There were even smaller ones.”

Tuesday was the beginning of the first mass burials of the victims of the suicide attacks in Sri Lanka, which killed more than 300 people on Sunday, including many children. Soldiers and an armored personnel carrier lined the roads as the burials took place.

Of the half-dozen sites simultaneously attacked on Sunday, the church in Negombo was the hardest hit. As many as 100 people were killed there.

On Tuesday, priests wearing crisp white robes trimmed with black sashes held funerals in a large tent just outside the church. The funerals were scheduled to go on all day. The neighborhood around the church had been turned into an enormous, fortified mourning ground, with hundreds of soldiers deployed in every direction and little white flags fluttering in the wind.

President Maithripala Sirisena in December. Mr. Sirisena’s government has given additional powers to the police and security forces to detain and interrogate people.CreditAdam Dean for The New York Times

Security officials facing dismissal

President Maithripala Sirisena said Tuesday that officials would lose their jobs over the failure to prevent the Easter Sunday bombings, despite warnings that attacks against churches were being planned.

Addressing Sri Lankans for the first time since the attacks, Mr. Sirisena pledged “major changes” in high-level positions in the security establishment within 24 hours. He said officials had been “negligent.”

“I must be truthful and admit that there were lapses on the part of defense officials,” Mr. Sirisena said in the televised address. “There was an intelligence report about the attack, but I was not kept informed.”

A senior presidential adviser, Shiral Lakthilaka, said that the secretary to the Ministry of Defense and the inspector general of the police were “earmarked for dismissal.” No replacements had been made so far, Mr. Lakthilaka said.

Mr. Sirisena said the military and the police would be restructured within a week. “In the next 24 hours, changes will be made to persons heading these institutions,” he said.

A memorial near Al Noor Mosque in Christchurch, New Zealand, for victims of the shootings there last month.CreditAdam Dean for The New York Times

Attack might have been in retaliation for Christchurch, government says

Sri Lankan officials raised the possibility on Tuesday that the bombers were hoping to avenge the killings of 50 Muslims in a shooting spree at two mosques in Christchurch, New Zealand, in March.

“The preliminary investigations have revealed that what happened in Sri Lanka was in retaliation for the attack against Muslims in Christchurch,” a junior defense minister, Ruwan Wijewardene, told Parliament.

Later Tuesday, Prime Minister Ranil Wickremesinghe put it more cautiously, saying that investigators were looking into a possible connection.

“It’s possible it could have been because of the Christchurch attacks,” he said. “We cannot say here.”

Neither official said what had led investigators to think that the bombings might have been retribution for the New Zealand massacre. And it was not clear how that theory aligned with warnings months earlier that Islamist radicals posed a serious threat and were stockpiling explosives and other weapons.

Officials said on Monday that a little-known Sri Lankan extremist group, National Thowheeth Jama’ath, had carried out the bombings.

On Tuesday, Mr. Wijewardene said that two local Islamist radical groups were involved: National Thowheeth Jama’ath and Jammiyathul Millathu Ibrahim. Again, Mr. Wickremesinghe was less definite, saying, “some investigations are underway on this other group.”

Government officials say the people who carried out the attacks were all Sri Lankan.

The police stood guard outside a safe house in Colombo that had been raided.CreditAdam Dean for The New York Times

The attackers’ leader may have been killed, and others are being hunted

The leader of National Thowheeth Jama’ath might have been one of the suicide bombers who were killed, the prime minister said, and “there are a few people on the run.”

At his news conference, Mr. Wickremesinghe was asked about the man police have identified as the leader of the group, Mohammed Zaharan.

“He is suspected to be one of the suicide bombers; they have to confirm it,” Mr. Wickremesinghe said. “Many of them have been identified, but not all.”

He added, “He was supposed to have led the group that went into one of the hotels.”

A fourth hotel was one of the suicide bomb targets, the prime minister said, but that attack failed. He did not name the hotel or say how the attack had gone wrong.

The government said on Tuesday that it had arrested 40 people in connection with the attacks, all of them Sri Lankans — an increase from the figure of 24 it gave on Monday.

As for identifying and capturing people all the people involved, Mr. Wickremesinghe said, “We are making progress, that’s all I could tell you.”

Melton Roy prayed at Sellakanda Catholic Cemetery in Negombo on Tuesday amid newly buried victims of the Easter Sunday bombing.CreditAdam Dean for The New York Times

National day of mourning as death toll rises

A full day of national mourning was declared across the country on Tuesday, as flags were lowered and a moment of silence was observed.

At 8:30 a.m., the time the first of six attacks were carried out on Sunday, Sri Lankans of differing faiths and ethnic groups bowed their heads and remained silent for three minutes.

Government officials said the number of people confirmed killed in the attacks was 321, up from 290 on Monday.

As part of the mourning period, liquor stores were ordered closed. Radio and television stations have played somber music throughout the day.

The front pages of local newspapers were similarly solemn on Tuesday. One, The Daily Mirror, printed a stark, all-black cover that read, “In remembrance of all those who lost their lives on 21.04.2019.”

‘Why wasn’t there any action?’ asks archbishop

Cardinal Malcolm Ranjith, the archbishop of Colombo, condemned the government on Tuesday for failing to act on an intelligence report that warned of a potential attack on churches.

“News media reported that there was information pertaining to a possible attack,” Cardinal Ranjith said at a news conference. “If that’s the case, couldn’t we have prevented the situation? Why wasn’t there any action?”

A security services briefing written at least 10 days before the bombings warned that National Thowheeth Jama’ath was planning to attack churches, but the government did not take action against the group. The prime minister, Mr. Wickremesinghe, and many of his cabinet ministers say the warning never reached them.

“If it was known, certainly we would have prevented many of the attacks,” the prime minister said.

“We have to look at the breakdown of security,” he said. “Some will have to be removed from their posts.”

Sri Lankan security personal were on high alert on Tuesday evening in Colombo.CreditAtul Loke/Getty Images

Government responds with a curfew, social media blackout and more

Sri Lankan officials took a series of extraordinary steps in an effort to keep control of their shaken country, aiming to prevent further extremist attacks and retaliatory violence.

President Sirisena said the government had given additional powers to the police and security forces to detain and interrogate people, and a curfew was imposed on Monday for the second day in a row, from 8 p.m. until 4 a.m.